1. Field of the Invention
The present invention relates to a method of resistance-welding laminates each having a basic three-layered structure consisting of metal layer/thermally softenable insulating layer/metal layer
2. Description of the Prior Art
Laminated materials obtained by combining a metallic material with various nonmetallic materials to improve various properties of the metallic material have been proposed. Examples of such laminated materials include a laminated material in which a synthetic resin film is formed, or is coated, on the surface of a metallic material so as to prevent corrosion of the metal; a laminated material for vibration prevention in which a thin layer of a vibration absorbing material such as rubber is formed inside a metallic material so as to provide good vibration prevention; and a light-weight laminated material consisting of an inner layer of a synthetic resin and a thin outer layer of a metal so as to provide a light-weight material with flexural stiffness equivalent to a metallic material. U.S. Pat. No. 4,313,996 (Feb. 2, 1982) discloses an example of a light-weight laminated material having a structure of metal/synthetic resin/metal. A structure having the following dimensions is disclosed: each metal skin layer is from about 2 to 20 mils (0.05-0.5 mm) thick, the ratio of the core thickness to skin thickness is less than 9:1, and the total laminate thickness is from about 5 to about 65 mils (0.13-1.65 mm). However, this reference does not describe the welding method.
The laminated metallic materials described above are all combined with electrically insulating materials such as synthetic resins. Therefore, two such laminated metallic materials cannot be welded by a conventional welding method since the overall structure is not conductive and so cannot be welded together.
In view of this, various improved welding methods have been proposed and actually practiced. FIG. 1 shows a welding method for projection welding laminates each having a metal/insulating material/metal structure. In this method, one laminate 1 consists of an internal insulating layer 4 sandwiched between metal layers 3a and 3b. Another laminate 2 consists of an internal insulating layer 6 sandwiched between metal layers 5a and 5b. These laminates 1 and 2 are to be welded together. Projection welding is performed such that electrodes 7 and 8 are brought into contact with the opposing metal layers 3b and 5a, and holding jigs 20a and 20b hold the upper and lower metal layers 3a and 5b. However, in a laminated metallic material obtained in this manner, only the metal layers 3b and 5a are welded. Metal layers 3a and 5b are not welded. Therefore, the welding strength is very weak. If the metal layer is a thin layer, the welded portion cannot serve a practical purpose.
FIG. 2 shows a known welding method utilizing a bypass circuit. According to this method, each of two laminates 9 and 10 to be welded has the following structure. The laminate 9 has an internal insulating layer 12 sandwiched between metal layers 11a and 11b. The laminate 10 consists of an internal insulating layer 14 sandwiched between metal layers 13a and 13b. A bypass circuit 15 is formed between the metal layers 11a and 13b. In the welding start period, a current flows through an electrode 16, the metal layer 11a, the bypass circuit 15, the metal layer 13b, and an electrode 17. Then, Joule heat is generated in the metal layers 11a and 13b and melts the internal insulating layers 12 and 14. According to this method, the melted portion of the insulating layers 12 and 14 near the electrode is made to flow transversely by the urging force of the electrodes 16 and 17. At the same time, the metal layers 11a and 13b are deformed and urged against the metal layers 11 b and 13a. As a result, a current flows directly between the electrodes 16 and 17 and the metal layers 11a, 11b, 13a and 13b to allow welding of the laminates. This method achieves excellent welding when applied to light-weight laminated materials in which the outer metal layers are thick and the internal insulating layers are thin. However, when this method is applied to a light-weight laminated material in which the outer metal layers are thin and the inner insulating layers are thick, problems are encountered. A light-weight laminated material of this type which can be welded by the method of the present invention is rendered light-weight by substituting the inner layer with a light material (normally insulating material) without decreasing the flexural stiffness. Such a light-weight laminated material is intended for use in a casing or the like. Therefore, deformation of the laminate at points other than the welding spot cannot be neglected.
Furthermore, in a light-weight laminate of this type, since the metal layer is thin, the outer metal layer is significantly damaged due to recessed deformation upon welding. Since the internal insulating layer generally consists of an organic material, it is decomposed and generates a gas at a melting point of the metal upon welding. This gas cannot diffuse out of the insulating layer due to the metal layers covering the insulating layer. The trapped gas then causes the problem of "doming". This tendency becomes particularly notable when the insulating layer is thick. However, when the method shown in FIG. 2 is adopted, the area in which the internal resin layer melts is not confined to the vicinity of the electrodes but also extends to the area connecting the electrodes and the bypass circuit. Therefore, the amount of resin which is squeezed by the urging forces of the electrodes is undesirably increased. In addition, a large amount of decomposition gas is produced. The laminate is distorted or waved within a wide area having the welded portion as its center. A good weld cannot, therefore, be obtained. In the method shown in FIG. 2, every time the welding spot is changed, the position of the bypass circuit must be changed and adjusted. This involves complex procedures. Furthermore, the shape of laminated materials which may be welded is limited due to the apparatus.
U.S. patent application Ser. No. 559,239 (June 21, 1966) discloses a method of welding conductors which are not coated with an insulator to a lead wire which is coated with an insulator. External heaters are connected to the electrodes to soften the insulators on the lead wire. Thus, the conductors and the lead wire are forcibly brought into contact with each other and are welded together. In this method, one part to be welded is the insulated wire, and the others are conductors which are not coated with an insulator. In such a welded article, the decomposition gas produced by heating the insulating material is freely discharged outside the article, so that a problem of "doming" is avoided.
U.S. Pat. No. 3,155,809 (Nov. 3, 1964) discloses a spot-welding or resistance-welding method. According to this method, when conductors coated with an insulator, particularly flexible ribbon-type cables, are electrically coupled, a pair of heated electrodes is used to clamp the conductors so as to soften the insulators. Then, the metal portions are physically brought into contact with each other. A welding current is made to flow between the electrodes to allow spot welding or resistance-welding. According to this method, as in the method disclosed in U.S. patent application Ser. No. 559,239, members to be welded are insulator coated conductors. With such members, the decomposition gas produced upon heating the insulators is freely discharged outside the structure, so that there is no doming problem.